To realize the active control of each segment of a segmented telescope, the figure control equation of hexagonal mirrors was built up, and the impact of sensor location on figure control system was studied. A working manner of the active control system based on the measurement requirements of relative segment locations was confirmed. Then, a figure control system consisting of 3 segments was built up, and the influence of the sensor location on figure control was analyzed.And also, a sensor setting scheme in which sensor is perpendicularly away in centimeters was put forward. Finally, the applicability of this scheme to the extent of segmented mirrors was testified. The analysis results indicate that the change of location between the mirrors can be exactly measured in fixed two sensors with 0.02 m stagger along the same gap when the segment mirror has side length of 0.9 m and diagonal length of 1.8 m,also the control system can give out the correct adjusted information of actuators. These conclutions reported here show that proposed scheme can satisfy the requirements of real time and high precise control of the active figure of the segmented mirror.